The present invention relates to an air fuel ratio control system for an engine equipped with a fuel vapor recovery system, and more specifically to an air fuel ratio control system designed to prevent degradation of the air fuel ratio control performance due to the fuel vapor recovery control.
There is known a system having a canister (adsorbing means) for adsorbing and trapping fuel evaporated in a fuel tank, a passage for purging and sucking the fuel vapor trapped in the canister by the aid of the intake vacuum and introducing the fuel vapor together with fresh air to an engine intake system. With these components, this system can prevent the fuel vapor from escaping from the fuel tank into the atmosphere. A Japanese Patent Provisional Publication No. 62-7962 discloses an fuel vapor adsorbing system of this kind.
If there is formed a crack in the piping for conveying the fuel vapor or if a flaw is developed in a sealed joint of pipes, the fuel vapor escapes through such a leak to the outside, and the system becomes deficient in controlling the evaporative emission.
Therefore, a Japanese Patent Provisional Publication No. 6-159158 proposes a leak diagnosis system for an evaporative emission control system. This diagnostic system has a drain cut valve for shutting off the supply of fresh air into the canister, and a purge cut valve disposed in a purge passage for communication between the canister and the engine intake system. By opening the purge cut valve while shutting off the supply of fresh air with the drain cut valve, the diagnostic system introduces the negative pressure into a line extending from the fuel tank through the canister to the engine, and detects a decreasing change of the pressure in the line caused by the opening of the purge cut valve. Thereafter, the diagnostic system detects an increasing change of the pressure in the line by closing the purge cut valve. From these pressure changes, this diagnostic system can diagnose a leak condition. (This Japanese Publication corresponds to a U.S. Pat. No. 5,408,866. The explanation and figures of this U.S. Patent about the leak diagnosis system is herein incorporated by reference.)